A dimmable discharge lamp lighting device is disclosed in U.S. Pat. No. 5,170,099. This disclosure is directed to provide a discharge lamp lighting device that can stably light a discharge lamp even at a low light flux level of less than 20% of its rated light illumination flux level. The disclosed discharge lamp lighting device includes a low-pressure mercury arc discharge lamp; a high frequency power supply for supplying a high frequency power to the discharge lamp; a dimming control circuit for carrying out a dimming of the discharge lamp from an arc discharge zone to a glow discharge zone; and a DC power supply that supplies to the discharge lamp a DC power at a level capable of maintaining discharge upon a low light flux dimming, the DC power being superposed on the high frequency power.
The above configuration enables stable dimming control of the light of the discharge lamp even at a low illumination level, without being extinguished or darkened under a normal condition.
However, the above conventional device has problems in that the vapor pressure of mercury in the discharge lamp is dependent upon a temperature, and thus the performance thereof is susceptible to the variation of ambient temperature. Especially, a low ambient temperature generally induces an increase in an equivalent impedance of the discharge lamp, which in turn results in the decrease in the DC power that is supplied to the discharge lamp. Consequently, a light flux from the discharge lamp is reduced, and therefore a flickering or an extinguishment of the lamp may occur.
Another dimmable discharge lamp lighting device is disclosed in Japanese Patent No. 3293650, which includes an inverter circuit with variable output for lighting a discharge lamp having a filament; a power detection unit for detecting a voltage in response to an output power of the inverter circuit; an output comparing unit for comparing the voltage detected by the power detection unit and an output reference voltage; a lamp voltage detection unit for detecting a voltage of the discharge lamp; a lamp voltage comparing unit for determining whether a voltage detected by the lamp voltage detection unit is higher than a lamp reference voltage; and an offset unit, in case where the voltage detected by the lamp voltage detection unit is determined to be higher than the lamp reference voltage, for reducing the voltage detected by the power detection unit relatively to the output reference voltage, whereby the reduced voltage is compared with the output reference voltage by the output comparing unit. The above lamp lighting device further includes a control unit. In a normal case, the control unit controls the output power of the inverter circuit depending on an output of the output comparing unit to stabilize the output power of the inverter circuit according to a preset lighting condition of the discharge lamp. However, in case where the lamp voltage detected by the lamp voltage detection unit is determined to be higher than the lamp reference voltage, the control unit controls the output power of the inverter circuit depending on the output of the output comparing unit while relatively reducing the voltage detected by the power detection unit by the offset unit.
In this way, if the voltage of the discharge lamp increases to be higher than the lamp reference voltage, the voltage detected in response to the output power of the inverter circuit is corrected to be lower than the actually detected level, enabling the output power of the inverter circuit to be increased in comparison with a case where the lamp voltage is not higher than the lamp reference voltage, which in turn prevents the discharge lamp from being extinguished.
Since the output of the inverter circuit is increased in case the lamp voltage is higher than the lamp reference voltage, the above conventional dimmable discharge lamp lighting device is considered to be able to prevent the discharge lamp from being extinguished and flickered when a current-voltage characteristic of the discharge lamp is within a negative domain. However, when the optical output of the discharge lamp is lowered down to equal to or less than 10% of the rated level for example, the current-voltage characteristic of the discharge lamp goes into a positive domain. In this case, since a lamp voltage decreases in company with a decrease of a lamp current, the conventional dimmable discharge lamp still suffers from extinguishment and flickering problems.